Nuclear tools have been used for several decades to determine the density of earth formations surrounding a borehole. Conventional density tools consist of a source of gamma-rays (or X-rays), at least one gamma-ray detector and shielding between the detector and the source, so that only scattered gamma-rays are detected. During density logging, gamma-rays from the tool source travel through the borehole, into the earth formation. The nuclear density tools rely on the Compton scattering of gamma-rays in the formation for the density measurements.
Due to size limitations and high gamma-ray intensity and energy requirements (more than about 500 keV for a monoenergetic source and an endpoint energy more than about 1 MeV for a Bremsstrahlung spectrum), downhole gamma-ray density tools have traditionally used radioactive chemical sources. However, the use of chemical sources creates a host of logistic and political issues. For example, there is a high level of liability associated with the handling and use of chemical sources. As a result, there are many governmental and safety controls required when handling, transporting, storing, and disposing of tools using chemical sources. Accordingly, there has been an effort in recent years to replace chemical sources with non-chemical, electronic sources (Bremsstrahlung).
While electrostatic machines can provide the required energy level, they are generally not suited for this borehole application. Likewise, linear RF machines can provide high intensity gamma-rays, however, their size and weight make them difficult to implement for borehole applications. In addition, they tend to be cost prohibitive. Induction machines, such as betatrons, are tempting non-chemical gamma-ray sources. However, the vacuum chambers of betatrons have been traditionally constructed of glass using hand made glass blowing techniques. This traditional manufacturing technique requires the employment of highly skilled artisans. Accordingly, betatrons of this type are not reproducible in a manner consistent enough for mass production. In addition, due to the many design problems (as described in part below), they have not been successfully implemented.
The circular shaped vacuum chamber and injector play vital roles in the successful operation of a borehole betatron. The chamber provides a vacuum environment wherein the electron beam accelerates. It is shaped like a donut that fits between two poles of the Betatron magnet and encompasses the center core of the magnet. Inside the chamber, a small electron gun, or injector, emits electrons at the beginning of, and in synchronization with, each acceleration cycle. A small fraction of the emitted electrons that fall within the magnet acceptance are trapped, accelerated to the full energy, and finally, directed toward a target, where some electrons collide with the target electrons. As a consequence of these collisions Bremsstrahlung radiation (x-rays) is emitted from the target. Most electrons emitted from the injector at the beginning of the acceleration cycle are not trapped and, lacking sufficient energy to penetrate the chamber wall, simply land on the inside surface of the chamber. On a bare insulating surface such as glass, excessive wall charge may lead to the premature disintegration of the accelerating beam due to the electrostatic field generated by the trapped charges. To alleviate this problem, the interior surface of the glass accelerator chamber is coated with a resistive layer having conductivity sufficient to bleed the wall charge to the ground without causing significant eddy current to retard the changing magnetic field. The appropriate resistivity of this layer is approximately 100-1000 Ω per square. The application of this resistive coating to traditional glass blown vacuum chambers has proven quite challenging. Accordingly, coating the inside of the accelerator chamber with an appropriate vacuum-compatible material that can survive electron beam bombardment is one of the many impediments to the development of a viable borehole betatron.
Only those electrons that fall within the magnet acceptance may be trapped and accelerated. Because magnet acceptance is generally very small, the injector alignment and position, which have significant impact on trapping efficiency, are very critical. The injector (at the back of which the target can be placed) is traditionally mounted at one end of a long cantilever arm, which consists of multiple conductive metal strips attached to the electrodes on the injector. The other ends of the metal strips are attached to a vacuum electrical feedthrough. The assembly is then inserted into the vacuum chamber through a long protruding port with a glass-to-metal joint and welded into place. The proper positioning and alignment of the injector attached at the end of a long cantilever arm inside the traditional glass blown vacuum chamber are very challenging. Accordingly, the mounting and alignment of the injector/target are two additional difficult design issues.
Proper operation of the betatron requires that the chamber be under vacuum. This presents additional challenges to the fabrication of the structure using traditional custom glass blown techniques. A second vacuum port must be provided in the glass structure to allow for the creation of a vacuum. The presence of these ports puts additional geometrical constraint on coil and magnet design.
Accordingly, it is an object of the present invention to provide a vacuum chamber design that is simpler to manufacture and has improved reproducibility.
It is another object of the present invention to provide a betatron vacuum chamber whose interior surface has the required conductivity.
It is yet another object of the present invention to provide a betatron vacuum chamber that allows simpler and more efficient alignment of the injector/target.